1. Field of the Invention
The present invention relates to a process for preparing titania nanorods for dye or quantum dot-sensitized solar cells; and, more particularly, to a process for preparing anatase type titania nanorods having the mean length of 100 nm to 300 nm by preparing aqueous titanium oxychloride (TiOCl2) solution by the use of titanium tetrachloride (TiCl4); preparing titanium oxyhydroxide [TiO(OH)2] precipitate with controlled particle size and shape via pH adjustment, control of reaction rate and heat-treatment; and adding hydrogen peroxide and water to titanium oxyhydroxide thus prepared and heating the mixture.
2. Description of Related Art
Titania (TiO2) is physically and chemically stable, having more than 2.5 of refractive index, which is higher than that of diamond (having the highest refractive index among natural materials). Higher refractive index increases the amount of light exited from the medium having lower refractive index in optical material, and decreases size of the core or thickness of the optical lense in an optical waveguide-type device. When particles of high refractive index are dispersed in polymeric medium, whiteness is improved due to their excellent ability of light scattering.
Titania, due to its high refractive property, is one of the most important industrial materials which have long been utilized as white pigment. Further, titania having high dielectric constant has been significantly considered as raw material for piezo electrics, dielectrics and semiconductor material, as electronic industry develops. Recently, use of titania rapidly extends to the applications in the field of cosmetics, thin layers, packing materials, paint, lubricant and fine ceramics as catalyst for removing organic contaminants, due to its chemical anti-corrosion property and photo-catalytic effect. Furthermore, extensive studies have been carried out for developing dye-sensitized solar cells utilizing titania as optical electrode and dye as solar photo-absorber, since they have much benefit in terms of cost in relation to efficiency.
A solar cell described herein means a cell wherein current-voltage is generated by using photovoltaic effect, generating electrons and holes by light absorbed by semiconductor. Initially, n-p diodes of inorganic semiconductors such as silicon and gallium arsenide (GaAs) have been employed as the semiconductor of solar cells. But the production cost was so high, thereby giving hindrance against vast use of solar cells.
In order to solve the problem, interests are concentrated on dye (or quantum-dot nanoparticle)-sensitized solar cells using cheap titania as the main constituent. A conventional dye-sensitized solar cell has a laminated structure consisting of a transparent substrate, a titania layer as photoanode, a photo-sensitized dye layer, an electrolyte layer, an electrode layer and a count electrode. The titania layer has porous structure in order to facilitate coloration of dye, and provides a pathway for electrons generated by reactions of electron- and hole-isolation.
Thus, titania should have high specific surface area and excellent ability for electron transfer itself, in order to prepare a dye-(or quantum nanoparticle) sensitized solar cell of high efficiency. Though essentially required is a technique for preparing nano-sized titania particles to obtain high specific surface area of titania, the problem to be most urgently solved for dye-(or quantum-dot nanoparticle)-sensitized solar cells of high efficiency at present is a technique to enhance the electron-transfer properties of titania. The electron-transfer properties of titania depend on shape of the particle, crystal structure, proportion of grain boundary, defects on the crystal structure, surface defects, or the like; being much affected by the process for preparing titania nanoparticles.
A variety of processes have been known for preparing titania to be used for dye-(or quantum-dot nanoparticle) sensitized solar cells or other use. However, conventional techniques for preparing titania via hydrolysis and condensation at ambient pressure and heat-treatment by using titanium tetrachloride or titanium oxychloride as starting material [Korean Patent Application No. 2000-0066290; U.S. Pat. No. 6,440,383; Bsaca et al., J. Am. Ceram. Soc., 79, 2185, 1996; C-C. Wang et al., Chem. Mater., 11, 3113, 1999; S. T. Aruna et al., J. Mater. Chem., 10, 2388, 2000; Y. Li et al., J. Mater. Chem., 12, 1387, 2002; J. Sun et al., J. Am. Ceram. Soc., 82, 927, 1999; W. Wang et al., J. Phys. Chem. B. 108, 14789, 2004] simply provide ball-shaped or rice-shaped titania with low aspect ratio. Certain processes for preparing rod-type titania having long major-axis (advantageously used for a dye-(or quantum-dot nanoparticle)-sensitized solar cell) have been reported by literature [A. Chemseddine et al, Eur. J. Inorg. Chem., 1999, 235, 1999; P. Davide CozzoliA et al, J. Am. Chem. Soc., 125, 14539, 2003]. However, in the processes, a large amount of surfactant (such as tetramethylammonium hydrocide and oleic acid) is used as growing agent for certain direction, so that washing or additional heat-treatment is required after preparing the rod-type titania.